Ramakumar, Rama “Electric Power Generation: Conventional Methods” The Electric Power Engineering pot

2 Electric Power Generation: Conventional Methods 2.1 Hydroelectric Power Generation Planning of Hydroelectric Facilities • Hydroelectric Plant Features • Special Considerations Affectin

Ramakumar, Rama “Electric Power Generation: Conventional Methods”

The Electric Power Engineering Handbook

Ed L.L Grigsby Boca Raton: CRC Press LLC, 2001

2 Electric Power

Generation:

Conventional Methods

Rama Ramakumar Oklahoma State University

2.1 Hydroelectric Power Generation Steven R Brockschink, James H Gurney, and Douglas B Seely

2 Electric Power Generation: Conventional Methods

2.1 Hydroelectric Power Generation

Planning of Hydroelectric Facilities • Hydroelectric Plant Features • Special Considerations Affecting Pumped Storage Plants • Commissioning of Hydroelectric Plants

2.2 Synchronous Machinery

General • Construction • Performance

2.3 Thermal Generating Plants

Plant Auxiliary System • Plant One-Line Diagram • Plant Equipment Voltage Ratings • Grounded vs Ungrounded Systems • Miscellaneous Circuits • DC Systems • Power Plant Switchgear • Auxiliary Transformers • Motors • Main Generator • Cable • Electrical Analysis • Maintenance and Testing • Start-Up

2.4 Distributed Utilities

Available Technologies • Fuel Cells • Microturbines • Combustion Turbines • Storage Technologies • Interface Issues • Applications

2.1 Hydroelectric Power Generation

Steven R Brockschink, James H Gurney, and Douglas B Seely

Hydroelectric power generation involves the storage of a hydraulic fluid, normally water, conversion ofthe hydraulic energy of the fluid into mechanical energy in a hydraulic turbine, and conversion of themechanical energy to electrical energy in an electric generator

The first hydroelectric power plants came into service in the 1880s and now comprise approximately22% (660 GW) of the world’s installed generation capacity of 3000 GW (Electric Power Research Institute,1999) Hydroelectricity is an important source of renewable energy and provides significant flexibility inbase loading, peaking, and energy storage applications While initial capital costs are high, the inherentsimplicity of hydroelectric plants, coupled with their low operating and maintenance costs, long servicelife, and high reliability, make them a very cost-effective and flexible source of electricity generation.Especially valuable is their operating characteristic of fast response for start-up, loading, unloading, andfollowing of system load variations Other useful features include their ability to start without theavailability of power system voltage (“black start capability”), ability to transfer rapidly from generationmode to synchronous condenser mode, and pumped storage application

Hydroelectric units have been installed in capacities ranging from a few kilowatts to nearly 1 GW.Multi-unit plant sizes range from a few kilowatts to a maximum of 18 GW

Hydroelectric Plant Schemes

There are three main types of hydroelectric plant arrangements, classified according to the method ofcontrolling the hydraulic flow at the site:

1 Run-of-the-river plants, having small amounts of water storage and thus little control of the flowthrough the plant

2 Storage plants, having the ability to store water and thus control the flow through the plant on adaily or seasonal basis

3 Pumped storage plants, in which the direction of rotation of the turbines is reversed during peak hours, pumping water from a lower reservoir to an upper reservoir, thus “storing energy”for later production of electricity during peak hours

off-Selection of Plant Capacity, Energy, and Other Design Features

The generating capacity of a hydroelectric plant is a function of the head and flow rate of water dischargedthrough the hydraulic turbines, as shown in Eq (2.1)

Another important planning consideration is the selection of the number and size of generating unitsinstalled to achieve the desired plant capacity and energy, taking into account installed unit costs, unitavailability, and efficiencies at various unit power outputs (American Society of Mechanical EngineersHydro Power Technical Committee, 1996)

Hydroelectric Plant Features

unit may have its shaft oriented in a vertical, horizontal, or inclined direction depending on the physical

Control of Small Hydroelectric Power Plants, 12 Copyright 1988 IEEE All rights reserved.)

for Control of Small Hydroelectric Power Plants, 13 Copyright 1988 IEEE All rights reserved.)

conditions of the site and the type of turbine applied Figure 2.1 shows a typical vertical shaft Francisturbine unit and Fig 2.2 shows a horizontal shaft propeller turbine unit The following sections willdescribe the main components such as the turbine, generator, switchgear, and generator transformer, aswell as the governor, excitation system, and control systems

In a reaction turbine, the water passes from a spiral casing through stationary radial guide vanes, throughcontrol gates and onto the runner blades at pressures above atmospheric There are two categories ofreaction turbine — Francis and propeller In the Francis turbine, installed at heads up to approximately

360 m, the water impacts the runner blades tangentially and exits axially The propeller turbine uses apropeller-type runner and is used at low heads — below approximately 45 m The propeller runner mayuse fixed blades or variable pitch blades (known as a Kaplan or double regulated type) which allows control

of the blade angle to maximize turbine efficiency at various hydraulic heads and generation levels Francisand propeller turbines may also be arranged in slant, tubular, bulb, and rim generator configurations.Water discharged from the turbine is directed into a draft tube where it exits to a tailrace channel,lower reservoir, or directly to the river

Flow Control Equipment

The flow through the turbine is controlled by wicket gates on reaction turbines and by needle nozzles

on impulse turbines A turbine inlet valve or penstock intake gate is provided for isolation of the turbineduring shutdown and maintenance

Spillways and additional control valves and outlet tunnels are provided in the dam structure to passflows that normally cannot be routed through the turbines

Generator

Synchronous generators and induction generators are used to convert the mechanical energy output ofthe turbine to electrical energy Induction generators are used in small hydroelectric applications (lessthan 5 MVA) due to their lower cost which results from elimination of the exciter, voltage regulator, andsynchronizer associated with synchronous generators The induction generator draws its excitation cur-rent from the electrical system and thus cannot be used in an isolated power system Also, it cannotprovide controllable reactive power or voltage control and thus its application is relatively limited.The majority of hydroelectric installations utilize salient pole synchronous generators Salient polemachines are used because the hydraulic turbine operates at low speeds, requiring a relatively largenumber of field poles to produce the rated frequency A rotor with salient poles is mechanically bettersuited for low-speed operation, compared to round rotor machines which are applied in horizontal axishigh-speed turbo-generators

Generally, hydroelectric generators are rated on a continuous-duty basis to deliver net kVA output at

a rated speed, frequency, voltage, and power factor and under specified service conditions including thetemperature of the cooling medium (air or direct water) Industry standards specify the allowabletemperature rise of generator components (above the coolant temperature) that are dependent on thevoltage rating and class of insulation of the windings (ANSI, C50.12-1982; IEC, 60034-1) The generatorcapability curve, Fig 2.3, describes the maximum real and reactive power output limits at rated voltagewithin which the generator rating will not be exceeded with respect to stator and rotor heating and otherlimits Standards also provide guidance on short circuit capabilities and continuous and short-timecurrent unbalance requirements (ANSI, C50.12-1982; IEEE, 492-1999)

Synchronous generators require direct current field excitation to the rotor, provided by the excitationsystem described in Section entitled “Excitation System” The generator saturation curve, Fig 2.4,describes the relationship of terminal voltage, stator current, and field current

While the generator may be vertical or horizontal, the majority of new installations are vertical Thebasic components of a vertical generator are the stator (frame, magnetic core, and windings), rotor (shaft,thrust block, spider, rim, and field poles with windings), thrust bearing, one or two guide bearings, upperand lower brackets for the support of bearings and other components, and sole plates which are bolted

to the foundation Other components may include a direct connected exciter, speed signal generator,rotor brakes, rotor jacks, and ventilation systems with surface air coolers (IEEE, 1095-1989)

The stator core is composed of stacked steel laminations attached to the stator frame The statorwinding may consist of single turn or multi-turn coils or half-turn bars, connected in series to form athree phase circuit Double layer windings, consisting of two coils per slot, are most common One ormore circuits are connected in parallel to form a complete phase winding The stator winding is normally

492-1999, IEEE Guide for Operation and Maintenance of Hydro-Generators, 16 Copyright 1999 IEEE All rights reserved.)

connected in wye configuration, with the neutral grounded through one of a number of alternativemethods which depend on the amount of phase-to-ground fault current that is permitted to flow (IEEE,C62.92.2-1989; C37.101-1993) Generator output voltages range from approximately 480 VAC to 22 kVACline-to-line, depending on the MVA rating of the unit Temperature detectors are installed between coils

in a number of stator slots

The rotor is normally comprised of a spider attached to the shaft, a rim constructed of solid steel orlaminated rings, and field poles attached to the rim The rotor construction will vary significantlydepending on the shaft and bearing system, unit speed, ventilation type, rotor dimensions, and charac-teristics of the driving hydraulic turbine Damper windings or amortisseurs in the form of copper orbrass rods are embedded in the pole faces, for damping rotor speed oscillations

and Maintenance of Hydro-Generators, 14 Copyright 1999 IEEE All rights reserved.)

The thrust bearing supports the mass of both the generator and turbine plus the hydraulic thrustimposed on the turbine runner and is located either above the rotor (“suspended unit”) or below therotor (“umbrella unit”) Thrust bearings are constructed of oil-lubricated, segmented, babbit-lined shoes.One or two oil lubricated generator guide bearings are used to restrain the radial movement of the shaft.Fire protection systems are normally installed to detect combustion products in the generator enclo-sure, initiate rapid de-energization of the generator and release extinguishing material Carbon dioxideand water are commonly used as the fire quenching medium

Excessive unit vibrations may result from mechanical or magnetic unbalance Vibration monitoringdevices such as proximity probes to detect shaft run-out are provided to initiate alarms and unit shutdown.The choice of generator inertia is an important consideration in the design of a hydroelectric plant.The speed rise of the turbine-generator unit under load rejection conditions, caused by the instantaneousdisconnection of electrical load, is inversely proportional to the combined inertia of the generator andturbine Turbine inertia is normally about 5% of the generator inertia During design of the plant, unitinertia, effective wicket gate or nozzle closing and opening times, and penstock dimensions are optimized

to control the pressure fluctuations in the penstock and speed variations of the turbine-generator duringload rejection and load acceptance Speed variations may be reduced by increasing the generator inertia

at added cost Inertia can be added by increasing the mass of the generator, adjusting the rotor diameter,

or by adding a flywheel The unit inertia also has a significant effect on the transient stability of theelectrical system, as this factor influences the rate at which energy can be moved in or out of the generator

to control the rotor angle acceleration during system fault conditions [see Chapter 11 — Power SystemDynamics and Stability and (Kundur, 1994)]

Generator Terminal Equipment

The generator output is connected to terminal equipment via cable, busbar, or isolated phase bus Theterminal equipment comprises current transformers (CTs), voltage transformers (VTs), and surge sup-pression devices The CTs and VTs are used for unit protection, metering and synchronizing, and forgovernor and excitation system functions The surge protection devices, consisting of surge arresters andcapacitors, protect the generator and low-voltage windings of the step-up transformer from lightningand switching-induced surges

of the automatic unit stopping sequence, or by operation of protective relay devices in the event of unitfault conditions

Generator Step-Up Transformer

The generator transformer steps up the generator terminal voltage to the voltage of the power system orplant switchyard Generator transformers are generally specified and operated in accordance with inter-national standards for power transformers, with the additional consideration that the transformer will

be operated close to its maximum rating for the majority of its operating life Various types of coolingsystems are specified depending on the transformer rating and physical constraints of the specific appli-cation In some applications, dual low-voltage windings are provided to connect two generating units to

a single bank of step-up transformers Also, transformer tertiary windings are sometimes provided toserve the AC station service requirements of the power plant

Excitation System

The excitation system fulfills two main functions:

1 It produces DC voltage (and power) to force current to flow in the field windings of the generator.There is a direct relationship between the generator terminal voltage and the quantity of currentflowing in the field windings as described in Fig 2.4

2 It provides a means for regulating the terminal voltage of the generator to match a desired setpoint and to provide damping for power system oscillations

Prior to the 1960s, generators were generally provided with rotating exciters that fed the generatorfield through a slip ring arrangement, a rotating pilot exciter feeding the main exciter field, and a regulatorcontrolling the pilot exciter output Since the 1960s, the most common arrangement is thyristor bridgerectifiers fed from a transformer connected to the generator terminals, referred to as a “potential sourcecontrolled rectifier high initial response exciter” or “bus-fed static exciter” (IEEE, 421.1-1986; 421.2-1990;421.4-1990; 421.5-1992) Another system used for smaller high-speed units is a brushless exciter with arotating AC generator and rotating rectifiers

Modern static exciters have the advantage of providing extremely fast response times and high fieldceiling voltages for forcing rapid changes in the generator terminal voltage during system faults This isnecessary to overcome the inherent large time constant in the response between terminal voltage andfield voltage (referred to as Tdo′, typically in the range of 5 to 10 sec) Rapid terminal voltage forcing isnecessary to maintain transient stability of the power system during and immediately after system faults.Power system stabilizers are also applied to static exciters to cause the generator terminal voltage to vary

in phase with the speed deviations of the machine, for damping power system dynamic oscillations [seeChapter 11 — Power System Dynamics and Stability and (Kundur, 1994)]

Various auxiliary devices are applied to the static exciter to allow remote setting of the generator voltageand to limit the field current within rotor thermal and under excited limits Field flashing equipment isprovided to build up generator terminal voltage during starting to the point at which the thyristors canbegin gating Power for field flashing is provided either from the station battery or alternating currentstation service

Governor System

The governor system is the key element of the unit speed and power control system (IEEE, 125-1988;IEC, 61362 [1998-03]; ASME, 29-1980) It consists of control and actuating equipment for regulatingthe flow of water through the turbine, for starting and stopping the unit, and for regulating the speedand power output of the turbine generator The governor system includes set point and sensing equipmentfor speed, power and actuator position, compensation circuits, and hydraulic power actuators whichconvert governor control signals to mechanical movement of the wicket gates (Francis and Kaplanturbines), runner blades (Kaplan turbine), and nozzle jets (Pelton turbine) The hydraulic power actuatorsystem includes high-pressure oil pumps, pressure tanks, oil sump, actuating valves, and servomotors.Older governors are of the mechanical-hydraulic type, consisting of ballhead speed sensing, mechanicaldashpot and compensation, gate limit, and speed droop adjustments Modern governors are of the electro-hydraulic type where the majority of the sensing, compensation, and control functions are performed

by electronic or microprocessor circuits Compensation circuits utilize proportional plus integral (PI) orproportional plus integral plus derivative (PID) controllers to compensate for the phase lags in thepenstock — turbine — generator — governor control loop PID settings are normally adjusted to ensurethat the hydroelectric unit remains stable when serving an isolated electrical load These settings ensurethat the unit contributes to the damping of system frequency disturbances when connected to anintegrated power system Various techniques are available for modeling and tuning the governor (WorkingGroup, 1992; Wozniak, 1990)

A number of auxiliary devices are provided for remote setting of power, speed, and actuator limits andfor electrical protection, control, alarming, and indication Various solenoids are installed in the hydraulicactuators for controlling the manual and automatic start-up and shutdown of the turbine-generator unit

Control Systems

Detailed information on the control of hydroelectric power plants is available in industry standards(IEEE, 1010-1987; 1020-1988; 1249-1996) A general hierarchy of control is illustrated in Table 2.1.Manual controls, normally installed adjacent to the device being controlled, are used during testing andmaintenance, and as a backup to the automatic control systems Figure 2.5 illustrates the relationship ofcontrol locations and typical functions available at each location Details of the control functions available

at each location are described in (IEEE, 1249-1996) Automatic sequences implemented for starting,synchronizing, and shutdown of hydroelectric units are detailed in (IEEE, 1010-1987)

Modern hydroelectric plants and plants undergoing rehabilitation and life extension are utilizingincreasing levels of computer automation (IEEE, 1249-1996; 1147-1991) The relative simplicity of hydro-electric plant control allows most plants to be operated in an unattended mode from remote control centers

Control Category Sub-Category Remarks Location Local Control is local at the controlled equipment or within sight of the equipment.

Centralized Control is remote from the controlled equipment, but within the plant.

Off Site Control location is remote from the project.

Mode Manual Each operation needs a separate and discrete initiation; could be applicable to any

of the three locations.

Automatic Several operations are precipitated by a single initiation; could be applicable to any

of the three locations

Operation (supervision)

Attended Operator is available at all times to initiate control action

Unattended Operation staff is not normally available at the project site.

Source: IEEE Standard 1249-1996, IEEE Guide for Computer-Based Control for Hydroelectric Power Plant Automation, 6 Copyright 1997 All rights reserved.

for Computer-Based Control for Hydroelectric Power Plant Automation, 7 With permission.)

An emerging trend is the application of automated condition monitoring systems for hydroelectricplant equipment Condition monitoring systems, coupled with expert system computer programs, allowplant owners and operators to more fully utilize the capacity of plant equipment and water resources,make better maintenance and replacement decisions, and maximize the value of installed assets

Protection Systems

The turbine-generator unit and related equipment are protected against mechanical, electrical, hydraulic,and thermal damage that may occur as a result of abnormal conditions within the plant or on the powersystem to which the plant is connected Abnormal conditions are detected automatically by means ofprotective relays and other devices and measures are taken to isolate the faulty equipment as quickly aspossible while maintaining the maximum amount of equipment in service Typical protective devicesinclude electrical fault detecting relays, temperature, pressure, level, speed, and fire sensors, and vibrationmonitors associated with the turbine, generator, and related auxiliaries The protective devices operate

in various isolation and unit shutdown sequences, depending on the severity of the fault

The type and extent of protection will vary depending on the size of the unit, manufacturer’s mendations, owner’s practices, and industry standards

recom-Specific guidance on application of protection systems for hydroelectric plants is provided in (IEEE,1010-1987; 1020-1988; C37.102-1995; C37.91-1985)

Plant Auxiliary Equipment

A number of auxiliary systems and related controls are provided throughout the hydroelectric plant tosupport the operation of the generating units (IEEE, 1010-1987; 1020-1988) These include:

1 Switchyard systems (see Chapter 5 — Substations)

2 Alternating current (AC) station service Depending on the size and criticality of the plant, multiplesources are often supplied, with emergency backup provided by a diesel generator

3 Direct current (DC) station service, normally provided by one or more battery banks, for supply

of protection, control, emergency lighting, and exciter field flashing

4 Lubrication systems, particularly for supply to generator and turbine bearings and bushings

5 Drainage pumps, for removing leakage water from the plant

6 Air compressors, for supply to the governors, generator brakes, and other systems

7 Cooling water systems, for supply to the generator air coolers, generator and turbine bearings,and step-up transformer

8 Fire detection and extinguishing systems

9 Intake gate or isolation valve systems

10 Draft tube gate systems

11 Reservoir and tailrace water level monitoring

12 Synchronous condenser equipment, for dewatering the draft tube to allow the runner to spin inair during synchronous condenser operation In this case, the generator acts as a synchronousmotor, supplying or absorbing reactive power

13 Service water systems

14 Overhead crane

15 Heating, ventilation, and air conditioning

16 Environmental systems

Special Considerations Affecting Pumped Storage Plants

A pumped storage unit is one in which the turbine and generator are operated in the reverse direction

to pump water from the lower reservoir to the upper reservoir The generator becomes a motor, drawingits energy from the power system, and supplies mechanical power to the turbine which acts as a pump.The motor is started with the wicket gates closed and the draft tube water depressed with compressed

air The motor is accelerated in the pump direction and when at full speed and connected to the powersystem, the depression air is expelled, the pump is primed, and the wicket gates are opened to commencepumping action

Pump Motor Starting

Various methods are utilized to accelerate the generator/motor in the pump direction during starting(IEEE, 1010-1987) These include:

1 Full voltage, across the line starting Used primarily on smaller units, the unit breaker is closedand the unit is started as an induction generator Excitation is applied near rated speed and machinereverts to synchronous motor operation

2 Reduced voltage, across the line starting A circuit breaker connects the unit to a starting bustapped from the unit step-up transformer at one third to one half rated voltage Excitation isapplied near rated speed and the unit is connected to the system by means of the generator circuitbreaker Alternative methods include use of a series reactor during starting and energization ofpartial circuits on multiple circuit machines

3 Pony motor starting A variable speed wound-rotor motor attached to the AC station service andcoupled to the motor/generator shaft is used to accelerate the machine to synchronous speed

4 Synchronous starting A smaller generator, isolated from the power system, is used to start themotor by connecting the two in parallel on a starting bus, applying excitation to both units, andopening the wicket gates on the smaller generator When the units reach synchronous speed, themotor unit is disconnected from the starting bus and connected to the power system

5 Semi-synchronous (reduced frequency, reduced voltage) starting An isolated generator is erated to about 80% rated speed and paralleled with the motor unit by means of a starting bus.Excitation is applied to the generating unit and the motor unit starts as an induction motor Whenthe speed of the two units is approximately equal, excitation is applied to the motor unit, bringing

accel-it into synchronism waccel-ith the generating unaccel-it The generating unaccel-it is then used to accelerate bothunits to rated speed and the motor unit is connected to the power system

6 Static starting A static converter/inverter connected to the AC station service is used to providevariable frequency power to accelerate the motor unit Excitation is applied to the motor unit atthe beginning of the start sequence and the unit is connected to the power system when it reachessynchronous speed The static starting system can be used for dynamic braking of the motor unitafter disconnection from the power system, thus extending the life of the unit’s mechanical brakes

Phase Reversing of the Generator/Motor

It is necessary to reverse the direction of rotation of the generator/motor by interchanging any two ofthe three phases This is achieved with multi-pole motor operated switches or with circuit breakers

Draft Tube Water Depression

Water depression systems using compressed air are provided to lower the level of the draft tube waterbelow the runner to minimize the power required to accelerate the motor unit during the transition topumping mode Water depression systems are also used during motoring operation of a conventionalhydroelectric unit while in synchronous condenser mode Synchronous condenser operation is used toprovide voltage support for the power system and to provide spinning reserve for rapid loading responsewhen required by the power system

Commissioning of Hydroelectric Plants

The commissioning of a new hydroelectric plant, rehabilitation of an existing plant, or replacement ofexisting equipment requires a rigorous plan for inspection and testing of equipment and systems andfor organizing, developing, and documenting the commissioning program (IEEE, 1248-1998)

IEC Standard 60034-1 (1996-12), Rotating Electrical Machines — Part 1: Rating and Performance.IEC Standard 61362 (1998-03), Guide to Specification of Hydraulic Turbine Control Systems.

IEEE Standard C37.91-1985 (Reaff 1990), IEEE Guide for Protective Relay Applications to Power Transformers.

IEEE Standard 421.1-1986 (Reaff 1996), IEEE Standard Definitions for Excitation Systems for nous Machines

Synchro-IEEE Standard 1010-1987 (Reaff 1992), IEEE Guide for Control of Hydroelectric Power Plants.IEEE Standard 125-1988 (Reaff 1996), IEEE Recommended Practice for Preparation of Equipment Spec- ifications for Speed-Governing of Hydraulic Turbines Intended to Drive Electric Generators.

IEEE Standard 1020-1988 (Reaff 1994), IEEE Guide for Control of Small Hydroelectric Power Plants.

IEEE Standard C62.92.2-1989 (Reaff 1993), IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part II — Grounding of Synchronous Generator Systems.

IEEE Standard 1095-1989 (Reaff 1994), IEEE Guide for Installation of Vertical Generators and ator/Motors for Hydroelectric Applications

Gener-IEEE Standard 421.2-1990, IEEE Guide for Identification, Testing and Evaluation of the Dynamic formance of Excitation Control Systems

Per-IEEE Standard 421.4-1990, IEEE Guide for the Preparation of Excitation System Specifications.IEEE Standard 1147-1991 (Reaff 1996), IEEE Guide for the Rehabilitation of Hydroelectric Power Plants.IEEE Standard 421.5-1992, IEEE Recommended Practice for Excitation Systems for Power Stability Studies.IEEE Standard C37.101-1993, IEEE Guide for Generator Ground Protection.

IEEE Standard C37.102-1995, IEEE Guide for AC Generator Protection.

IEEE Standard 1249-1996, IEEE Guide for Computer-Based Control for Hydroelectric Power Plant Automation.IEEE Standard 1248-1998, IEEE Guide for the Commissioning of Electrical Systems in Hydroelectric Power Plants

IEEE Standard 492-1999, IEEE Guide for Operation and Maintenance of Hydro-Generators.

Kundur, P., Power System Stability and Control, McGraw-Hill, New York, 1994

Working Group on Prime Mover and Energy Supply Models for System Dynamic Performance Studies,Hydraulic turbine and turbine control models for system dynamic studies, IEEE Trans Power Syst., 7(1), February 1992

Wozniak, L., Graphical Approach to Hydrogenerator Governor Tuning, IEEE Trans Energy Conv., 5(3),September 1990

Synchronous motors, generators, and condensers perform similarly, except for a heavy cage winding

on the rotor of motors and condensers for self-starting

A rotor has physical magnetic poles, arranged to have alternating north and south poles around therotor diameter which are excited by electric current, or uses permanent magnets, having the same number

of poles as the stator electromagnetic poles

The rotor RPM = 120 × Electrical System Frequency/Poles

The stator winding, fed from external AC multi-phase electrical power, creates rotating electromagneticpoles

At speed, rotor poles turn in synchronism with the stator rotating electromagnetic poles, torque beingtransmitted magnetically across the “air gap” power angle, lagging in generators and leading in motors

Synchronous machine sizes range from fractional watts, as in servomotors, to 1500 MW, as in largegenerators

Voltages vary, up to 25,000 V AC stator and 1500 V DC rotor

Installed horizontal or vertical at speed ranges up to 130,000 RPM, normally from 40 RPM (waterwheelgenerators) to 3600 RPM (turbine generators)

Frequency at 60 or 50 Hz mostly, 400 Hz military; however, synthesized variable frequency electricalsupplies are increasingly common and provide variable motor speeds to improve process efficiency

Typical synchronous machinery construction and performance are described; variations may exist onspecial smaller units

This document is intentionally general in nature Should the reader want specific application mation, refer to standards: NEMA MG-1; IEEE 115, C50-10 and C50-13; IEC 600034: 1-11,14-16,18, 20,

infor-44, 72, and 136, plus other applicable specifications

Construction (See Fig 2.6 )

Stator

Frame

The exterior frame, made of steel, either cast or a weldment, supports the laminated stator core and hasfeet, or flanges, for mounting to the foundation Frame vibration from core magnetic forcing or rotorunbalance is minimized by resilient mounting the core and/or by designing to avoid frame resonancewith forcing frequencies If bracket type bearings are employed, the frame must support the bearings,oil seals, and gas seals when cooled with hydrogen or gas other than air The frame also provides protectionfrom the elements and channels cooling air, or gas, into and out of the core, stator windings, and rotor

When the unit is cooled by gas contained within the frame, heat from losses is removed by coolers havingwater circulating through finned pipes of a heat exchanger mounted within the frame Where coolingwater is unavailable and outside air cannot circulate through the frame because of its dirty or toxiccondition, large air-to-air heat exchangers are employed, the outside air being forced through the cooler

by an externally shaft-mounted blower

Stator Core Assembly

The stator core assembly of a synchronous machine is almost identical to that of an induction motor Amajor component of the stator core assembly is the core itself, providing a high permeability path formagnetism The stator core is comprised of thin silicon steel laminations and insulated by a surfacecoating minimizing eddy current and hysteresis losses generated by alternating magnetism The lamina-tions are stacked as full rings or segments, in accurate alignment, either in a fixture or in the stator frame,having ventilation spacers inserted periodically along the core length The completed core is compressedand clamped axially to about 10 kg/cm2 using end fingers and heavy clamping plates Core end heatingfrom stray magnetism is minimized, especially on larger machines, by using non-magnetic materials atthe core end or by installing a flux shield of either tapered laminations or copper shielding

A second major component is the stator winding made up of insulated coils placed in axial slots ofthe stator core inside diameter The coil make-up, pitch, and connections are designed to produce rotatingstator electromagnetic poles in synchronism with the rotor magnetic poles The stator coils are retainedinto the slots by slot wedges driven into grooves in the top of the stator slots Coil end windings arebound together and to core-end support brackets If the synchronous machine is a generator, the rotatingrotor pole magnetism generates voltage in the stator winding which delivers power to an electric load

If the synchronous machine is a motor, its electrically powered stator winding generates rotating tromagnetic poles and the attraction of the rotor magnets, operating in synchronism, produces torqueand delivery of mechanical power to the drive shaft

elec-Rotor

The Rotor Assembly

The rotor of a synchronous machine is a highly engineered unitized assembly capable of rotating factorily at synchronous speed continuously according to standards or as necessary for the application

satis-The central element is the shaft, having journals to support the rotor assembly in bearings Located atthe rotor assembly axial mid-section is the rotor core embodying magnetic poles When the rotor isround it is called “non-salient pole”, or turbine generator type construction and when the rotor hasprotruding pole assemblies, it is called “salient pole” construction

The non-salient pole construction, used mainly on turbine generators (and also as wind tunnel fandrive motors), has two or four magnetic poles created by direct current in coils located in slots at therotor outside diameter Coils are restrained in the slots by slot wedges and at the ends by retaining rings

on large high-speed rotors, and fiberglass tape on other units where stresses permit This construction

is not suited for use on a motor requiring self-starting as the rotor surface, wedges, and retaining ringsoverheat and melt from high currents of self-starting

A single piece forging is sometimes used on salient pole machines, usually with four or six poles.

Salient poles can also be integral with the rotor lamination and can be mounted directly to the shaft orfastened to an intermediate rotor spider Each distinct pole has an exciting coil around it carrying

synchronous motor (From The ABC’s of Synchronous Motors, 7(1), 5, 1944 The Electric Machinery Company, Inc.

With permission.)

excitation current or else it employs permanent magnets In a generator, a moderate cage winding in theface of the rotor poles, usually with pole-to-pole connections, is employed to dampen shaft torsionaloscillation and to suppress harmonic variation in the magnetic waveform In a motor, heavy bars andend connections are required in the pole face to minimize and withstand the high heat of starting duty

Direct current excites the rotor windings of salient, and non-salient pole motors and generators, exceptwhen permanent magnets are employed The excitation current is supplied to the rotor from either anexternal DC supply through collector rings or a shaft-mounted brushless exciter Positive and negativepolarity bus bars or cables pass along and through the shaft as required to supply excitation current tothe windings of the field poles

When supplied through collector rings, the DC current could come from a shaft-driven DC or ACexciter rectified output, from an AC-DC motor-generator set, or from plant power DC current supplied

by a shaft-mounted AC generator is rectified by a shaft-mounted rectifier assembly

As a generator, excitation current level is controlled by the voltage regulator As a motor, excitation current

is either set at a fixed value, or is controlled to regulate power factor, motor current, or system stability

In addition, the rotor also has shaft-mounted fans or blowers for cooling and heat removal from theunit plus provision for making balance weight additions or corrections

Bearings and Couplings

Bearings on synchronous machinery are anti-friction, grease, or oil-lubricated on smaller machines,journal type oil-lubricated on large machines, and tilt-pad type on more sophisticated machines, espe-cially where rotor dynamics are critical Successful performance of magnetic bearings, proving to besuccessful on turbo-machinery, may also come to be used on synchronous machinery as well

As with bearings on all large electrical machinery, precautions are taken with synchronous machines toprevent bearing damage from stray electrical shaft currents An elementary measure is the application ofinsulation on the outboard bearing, if a single-shaft end unit, and on both bearing and coupling at the sameshaft end for double-shaft end drive units Damage can occur to bearings even with properly appliedinsulation, when solid-state controllers of variable frequency drives, or excitation, cause currents at highfrequencies to pass through the bearing insulation as if it were a capacitor Shaft grounding and shaft voltageand grounding current monitoring can be employed to predict and prevent bearing and other problems

Performance

Synchronous Machines, in General

This section covers performance common to synchronous motors, generators, and condensers

Saturation curves (Fig 2.7) are either calculated or obtained from test and are the basic indicators ofmachine design suitability From these the full load field, or excitation, amperes for either motors or

generators are determined as shown, on the rated voltage line, as “Rated Load.” For synchronous

con-densers, the field current is at the crossing of the zero P.F saturation line at 1.0 V As an approximatemagnetic figure of merit, the no-load saturation curve should not exceed its extrapolated straight line

by more than 25%, unless of a special design From these criteria, and the knowledge of the stator currentand cooling system effectiveness, the manufacturer can project the motor component heating, and thusinsulation life, and the efficiency of the machine at different loads

Vee curves (Fig 2.8) show overall loading performance of a synchronous machine for different loadsand power factors, but more importantly show how heating and stability limit loads For increasedhydrogen pressures in a generator frame, the load capability increases markedly

The characteristics of all synchronous machines when their stator terminals are short-circuited aresimilar (see Fig 2.9) There is an initial subtransient period of current increase of 8 to 10 times rated,with one phase offsetting an equal amount These decay in a matter of milliseconds to a transient value

of 3 to 5 times rated, decaying in tenths of a second to a relatively steady value Coincident with this, thefield current increases suddenly by 3 to 5 times, decaying in tenths of a second The stator voltage onthe shorted phases drops to zero and remains so until the short circuit is cleared

Synchronous Generator Capability

The synchronous generator normally has easy starting duty as it is brought up to speed by a prime mover.Then the rotor excitation winding is powered with DC current, adjusted to rated voltage, and transferred

to voltage regulator control It is then synchronized to the power system, closing the interconnectingcircuit breaker as the prime mover speed is advancing, at a snail’s pace, leading the electric system Once

on line, its speed is synchronized with the power system and KW is raised by increasing the prime mover

KW input The voltage regulator adjusts excitation current to hold voltage Increasing the voltage regulator